Expression and functional characterization of GABA transporters in crayfish neurosecretory cells

Molecular cloning, functional characterization and differential expression of two novel GABAAR-like subunits from red swamp crayfish Procambarus clarkii

In this work, we cloned and functionally expressed two novel GABAA receptor subunits from Procambarus clarkii crayfish. These two new subunits, PcGABAA-α and PcGABAA-β2, revealed significant sequence homology with the PcGABAA-β subunit, previously identified in our laboratory. In addition, PcGABAA-α subunit also shared a significant degree of identity with the Drosophila melanogaster genes DmGRD (GABA and glycine-like receptor subunits of Drosophila) as well as PcGABAA-β2 subunit with DmLCCH3 (ligand-gated chloride channel homolog 3). Electrophysiological recordings showed that the expression in HEK cells of the novel subunits, either alone or in combination, failed to form functional homo- or heteromeric receptors. However, the co-expression of PcGABAA-α with PcGABAA-β evoked sodium- or chloride-dependent currents that accurately reproduced the time course of the GABA-evoked currents in the X-organ neurons from crayfish, suggesting that these GABA subunits combine to form two types of GABA receptors, one with cationic selectivity filter and the other preferentially permeates anions. On the other hand, PcGABAA-β2 and PcGABAA-β co-expression generated a chloride current that does not show desensitization. Muscimol reproduced the time course of GABA-evoked currents in all functional receptors, and picrotoxin blocked these currents; bicuculline did not block any of the recorded currents. Reverse transcription polymerae chain reaction (RT-PCR) amplifications and FISH revealed that PcGABAA-α and PcGABAA-β2 are predominantly expressed in the crayfish nervous system. Altogether, these findings provide the first evidence of a neural GABA-gated cationic channel in the crayfish, increasing our understanding of the role of these new GABAA receptor subunits in native heteromeric receptors.

Enhancement of muscle contraction in the stomach of the crab Cancer borealis: a possible hormonal role for GABA

Gamma-aminobutyric acid (GABA) is best known as an inhibitory neurotransmitter in the mammalian central nervous system. Here we show, however, that GABA has an excitatory effect on nerve-evoked contractions and on excitatory junctional potentials (EJPs) of the gastric mill 4 (gm4) muscle from the stomach of the crab Cancer borealis. The threshold concentration for these effects was between 1 and 10 micromol l(-1). Using immunohistochemical techniques, we found that GABA is colocalized with the vesicle-associated protein synapsin in nearby nerves and hence is presumably released there. However, since these nerves do not innervate the muscle directly, we conclude that these release sites are not the likely source of the GABA responsible for muscle modulation. We also extracted hemolymph from the crab pericardial cavity, which contains the pericardial organs, a major neurosecretory structure. Through reversed-phase liquid chromatography-mass spectrometry analysis we determined the concentration of GABA in the hemolymph to be 3.3 +/- 0.7 micromol l(-1), high enough to modulate the muscle. These findings suggest that the gm4 muscle could be modulated by GABA produced by and released from a distant neurohemal organ.

The GABAergic-like system in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite) (Cirripedia, Crustacea)

In the present study, biochemical and immunochemical methods were used to investigate the presence and distribution of GABA, glutamate decarboxylase (GAD), GABA(B)R1 and GABA(A) gamma2 subunit receptors and the vesicular GABA transporter (vGAT) in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite). GAD(65/67) immunoreactive neuron cell bodies and nerve fibers were detected in the central nervous system. Paired GAD(65/67) immunoreactive nerves running from the posterior ganglion to the body and limb muscles were detected. Thin GABA-immunoreactive nerve terminals were present on striated muscular fibers and in the antennules. Furthermore, GABA, GAD(65/67), GABA(B)R1 and GABA(A)gamma2 subunit receptors and vGAT were observed in the lateral compound eyes, and GABA(A)gamma2 subunit receptor immunoreactivity was seen in the naupliar eye. These results suggest a neurotransmitter/neuromodulatory role for GABA in thoracic muscle contraction and regulatory functions in compound eyes and antennules of B. amphitrite cyprids.

Histamine operates Cl–-gated channels in crayfish neurosecretory cells

We describe a histamine-activated Cl– conductance in the X-organ neurons from crayfish Cherax quadricarinatus, which has comparable properties to the homomultimeric histamine-gated ion channels described in Drosophila. Topical application of histamine inhibited spontaneous neuronal firing in the X-organ sinus gland tract, concomitant with an increase in the membrane conductance. In X-organ neurons in culture and under voltage-clamp conditions, histamine evoked outward currents at –40 mV that reversed at the Cl– equilibrium potential. Histamine sensitivity in these neurons had a half-maximal response(EC50)=3.3±1 μmol l–1, with a Hill number of 2.6±0.4. The histamine-evoked current was blocked by tiotidine, cimetidine, ranitidine and 256±11 and 483±11 μmol l–1, respectively) and d-tubocurarine(IC50=21±2 μmol l–1), but was insensitive to picrotoxin, bicuculline and strychnine. Neither GABA nor glutamate was capable of desensitizing the histamine response, indicating that histamine activates a particular Cl– conductance. The presence of immunoreactive neurons to histamine in the medulla terminalis with axonal projections to the neuropile suggests a possible histaminergic modulation of the X-organ sinus gland system.

γ-Aminobutyric acid (GABA) signaling components inDrosophila: Immunocytochemical localization of GABABreceptors in relation to the GABAAreceptor subunit RDL and a vesicular GABA transporter

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in insects and is widely distributed in the central nervous system (CNS). GABA acts on ion channel receptors (GABA(A)R) for fast inhibitory transmission and on G-protein-coupled ones (GABA(B)R) for slow and modulatory action. We used immunocytochemistry to map GABA(B)R sites in the Drosophila CNS and compared the distribution with that of the GABA(A)R subunit RDL. To identify GABAergic synapses, we raised an antiserum to the vesicular GABA transporter (vGAT). For general GABA distribution, we utilized an antiserum to glutamic acid decarboxylase (GAD1) and a gad1-GAL4 to drive green fluorescent protein. GABA(B)R-immunoreactive (IR) punctates were seen in specific patterns in all major neuropils of the brain. Most abundant labeling was seen in the mushroom body calyces, ellipsoid body, optic lobe neuropils, and antennal lobes. The RDL distribution is very similar to that of GABA(B)R-IR punctates. However, the mushroom body lobes displayed RDL-IR but not GABA(B)R-IR material, and there were subtle differences in other areas. The vGAT antiserum labeled punctates in the same areas as the GABA(B)R and appeared to display presynaptic sites of GABAergic neurons. Various GAL4 drivers were used to analyze the relation between GABA(B)R distribution and identified neurons in adults and larvae. Our findings suggest that slow GABA transmission is very widespread in the Drosophila CNS and that fast RDL-mediated transmission generally occurs at the same sites.

An improved method for long-term measuring of hemolymph fluctuations of non-essential amino acids, GABA and histamine from freely moving crayfish

The microdialysis method was adapted to obtain long-term hemolymph dialysates from the pericardial cavity of freely moving Procambarus clarkii crayfish, to measure fluctuations of non-essential amino acids, GABA and histamine by high-performance liquid chromatography using off-line fluorometric derivatization. Asp, Ala, Tau, GABA and histamine (HA) reached its maximal concentrations at the daybreak, whereas glutamate (Glu), Gln and Gly peaked at the end of the light period. The minimum and maximal detected amounts for each substance along the 24h cycle were (in microM): 20-300Asp, 100-200Glu, 400-700Gln, 400-600Gly, 100-200Tau, 150-300Ala, 2-10 GABA and 25-250HA. Cocktails containing the relative concentration of each amino acid, GABA and histamine resulted in a hyperpolarization that reduced the spontaneous firing of cultured peptidergic X organ neurons. Glu, GABA and histamine evoked a long-lasting hyperpolarization that suppressed the spontaneous firing, whereas Asp, Gly and Tau evoked a depolarization accompanied with neuronal firing. Finally, neither Ala nor Gln modified the resting membrane potential.

Subcellular redistribution of the renal betaine transporter during hypertonic stress

The betaine transporter (BGT1) protects cells in the hypertonic renal inner medulla by mediating uptake and accumulation of the osmolyte betaine. Transcriptional regulation plays an essential role in upregulation of BGT1 transport when renal cells are exposed to hypertonic medium for 24 h. Posttranscriptional regulation of the BGT1 protein is largely unexplored. We have investigated the distribution of BGT1 protein in live cells after transfection with BGT1 tagged with enhanced green fluorescent protein (EGFP). Fusion of EGFP to the NH2 terminus of BGT1 produced a fusion protein (EGFP-BGT) with transport properties identical to normal BGT1, as determined by ion dependence, inhibitor sensitivity, and apparent Km for GABA. Confocal microscopy of EGFP-BGT fluorescence in transfected Madin-Darby canine kidney (MDCK) cells showed that hypertonic stress for 24 h induced a shift in subcellular distribution from cytoplasm to plasma membrane. This was confirmed by colocalization with anti-BGT1 antibody staining. In fibroblasts, transfected EGFP-BGT caused increased transport in response to hypertonic stress. The activation of transport was not accompanied by increased expression of EGFP-BGT, as determined by Western blotting. Membrane insertion of EGFP-BGT protein in MDCK cells began within 2-3 h after onset of hypertonic stress and was blocked by cycloheximide. We conclude that posttranscriptional regulation of BGT1 is essential for adaptation to hypertonic stress and that insertion of BGT1 protein to the plasma membrane may require accessory proteins.